The Home Secretary has rejected calls to rethink a planned ban on the drug khat, which is most commonly used by people of East African origin including a sizable Somali community in the West Midlands.

It follows warnings from MPs that there was no evidence the drug is harmful – and police will be forced to target specific immigrant communities if the drug is outlawed, risking “antagonism or friction”.

The West Midlands has the second highest concentration of people of Somali origin outside London, followed by Bristol, Greater Manchester and Leicester.

A Home Office survey of people of Somali origin in Birmingham in 2005 found that 34 per cent of the overall sample said they had chewed khat in the month before the interview.

The Commons Home Affairs Committee, which includes MPs David Winnick (Lab Walsall North) and Ian Austin (Lab Dudley North), had urged Ministers to rethink plans to classify khat as a class C drug, making the importation, possession and supply of khat a criminal offence.

But Theresa May, the Home Secretary, has now issued her response – and insisted she sees no reason to change her mind.

Khat is the common name for the leaves, stems and shoots of the plant of the species Catha edulis and is chewed in a social setting, typically at home, at parties and in khat cafes.

The plant is native to Africa and the Middle East and is cultivated commercially in Ethiopia, Kenya and Yemen. An estimated 90,000 people use khat in the UK and its consumption is confined almost exclusively to the Somali, Yemeni and Ethiopian communities.

Ms May presented changes to the Misuse of Drugs Act 1971 to the Commons in October last year to criminalise khat, and these are expected to be approved in the near future.

The West Midlands conurbation was home to 8,517 people born in Somalia at the time of the 2011 census, the largest number of any built-up area outside greater London.

But the Home Affairs committee has warned that the decision to make khat illegal was not based on evidence that it causes medical or social harm – and the committee said it had not received any convincing evidence that the consumption of khat was harmful.

The MPs warned: “Controlling khat will create a crime which is only likely to be committed by members of certain specific communities, who already experience a degree of marginalisation within the UK.

“Enforcing that control will involve policing an activity that is carried out by a very small proportion of the population, all of whom belong to two or three diaspora communities, and a disproportionate number of whom are first-generation immigrants.

“To do this sensitively, in a way that does not create antagonism or friction between the police and the communities concerned, will present a very significant policing challenge.

“There is a high risk of alienating those who have until now pursued a perfectly lawful social activity, which could have a consequential impact on a wide range of police and law enforcement activity.”

But in a response, Ms May said: “I have made clear that the decision to ban khat was finely balanced.

“Having reviewed the Committee’s report, I am not persuaded to reconsider my decision which was made following an extensive consultation process and careful consideration, not least because of the breadth and complexity of the issues associated with khat in the UK and abroad.”